1. Field of the Invention
The present invention relates to an optical wavelength-division multiplexing (WDM) access system which transmits optical signals between a center unit (OSU; Optical Subscriber Unit) and a plurality of optical network units (ONUs) in both directions.
Priority is claimed on Japanese Patent Application No. 2002-378079, filed Dec. 26, 2002, the content of which is incorporated herein by reference.
2. Description of Related Art
As one embodiment of an optical multiplexing access system, an optical wavelength-division multiplexing (WDM) access system in which each of the ONUs occupies a different wavelength upon a star type optical access line which multiplexes and demultiplexes a plurality of signals via wavelength splitters is investigated.
FIG. 14 shows a structural example of a conventional optical wavelength-division multiplexing access system (which is disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-196536, hereinafter termed Patent Reference 1). Here, by way of example, a single wavelength band λd is assigned for transmission of downstream signals from the OSU to the ONUs, and a single wavelength band λu (≠λd) is assigned for transmission of upstream signals from the ONUs to the OSU, and moreover respective wavelengths λd1 through λdn of the wavelength band λd and respective wavelengths λu1 through λun of the wavelength band λu are assigned to the respective ONUs.
A transmitting section 51 of the OSU 50 performs wavelength-division multiplexing of the downstream optical signal of the wavelength band λd (the wavelengths λd1 through λdn) and of the optical carrier for upstream transmission of the wavelength band λu (the wavelengths λu1 through λun), and transmits them to a wavelength splitter 60 via an optical fiber transmission line 1. The wavelength splitter 60 demultiplexes the downstream optical signal of the wavelength band λd and the optical carrier for upstream transmission of the wavelength band λu, and transmits the downstream optical signal of the wavelengths λd1 through λdn and the optical carriers for upstream transmission of the wavelengths λu1 through λun to respectively corresponding ONUs 70-1 through 70-n via optical fiber transmission lines 3.
The ONU 70-1 demultiplexes the downstream optical signal of wavelength λd1 which has been transmitted and has arrived and the optical carrier for upstream transmission of wavelength λu1 with a WDM coupler 71, and receives the downstream optical signal of wavelength λd1 with an optical receiver 32, while it modulates the optical carrier for upstream transmission of wavelength λu1 with an optical modulator 73, and transmits the modulated optical signal via an optical fiber transmission line 4 to the wavelength splitter 60 as an upstream optical signal. The same processes are performed by the other ONUs. The upstream optical signals of wavelengths λu1 through λun which have been transmitted from the respective ONUs are wavelength-division multiplexed by the wavelength splitter 60, then are transmitted to the OSU 50 via an optical fiber transmission line 2 for upstream transmission, and are received by a receiving section 52 thereof.
Here, as shown in FIG. 14, the wavelength band λd (the wavelengths λd1 through λdn) of the downstream optical signal and the wavelength band λu (the wavelengths λu1 through λun) of the optical carrier for upstream transmission are disposed so that they do not overlap in the wavelength domain. The array waveguide grating (AWG) which is used as the wavelength splitter 60 has the characteristic that it can multiplex and demultiplex wavelengths of a FSR (free spectral range) spacing simultaneously. Due to the characteristic of FSR, it is possible to demultiplex the downstream signal wavelengths (for example λd1) and the upstream signal wavelengths (for example λu1) upon the same port. With this exemplary conventional technique, it is anticipated that, by taking advantage of this function, it will be possible to standardize the structural elements of the respective ONUs (i.e. to reduce the number of different types of component incorporated in them). In other words, it is possible to demultiplex the downstream optical signal and the optical carrier for upstream transmission by utilizing a WDM coupler 71 of the same specification at each ONU for demultiplexing the wavelength band λd and the wavelength band λu, and to ensure that the optical carriers for upstream transmission do not induce any interference in the optical receivers 72.
It should be understood that a method may also be proposed of, when transmitting the optical carriers for upstream of the wavelengths λu1 through λun from the transmitting section 51 of the OSU 50 as broadband light which includes the wavelengths λu1 through λun, spectrum slicing with the wavelength splitter 60 into the optical carriers for upstream transmission of wavelengths λu1 through λun and transmitting to each ONU (this is disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-177505, hereinafter termed Patent Reference 2).
In this connection, this type of expedient takes as its objective to standardize the structural elements of the ONUs 70-1 through 70-n as described above (i.e. to reduce the number of different types of component incorporated in them). In other words, by first supplying the optical carrier for upstream transmission of each wavelength from the OSU 50 to each ONU, it is possible to increase the compatibility without it being necessary to incorporate individual light sources of the respective wavelengths which are respectively assigned to the ONUs. Next, it is possible to increase the compatibility of the WDM couplers 71 in the respective ONUs by dividing the wavelength band λd for the downstream signals from the wavelength band λu for the upstream signals by taking advantage of the functions of the AWG as described above.
Furthermore, in Japanese Unexamined Patent Application, First Publication No. Heisei 8-8878 (hereinafter termed Patent Reference 3) and in Japanese Patent Application No. 2002-231632 (published as Japanese Unexamined Patent Application, First Publication No. 2003-134058 and hereinafter termed Patent Reference 4), it has been proposed to use a single type of ONU and thus to reduce the manufacturing cost by implementing a spectrum slicing scheme.
Apart from the above, as a one-way point to point transmission system, there is an example which applies a spectrum slicing scheme by incorporating and using a wide spectrum light source and a single input single output type optical band-pass filter in its light source section (see Japanese Unexamined Patent Application, First Publication No. Hei 7-177127, hereinafter termed Patent Reference 5).
Furthermore, as shown in FIG. 15, there has also been proposed a structure in which, in a construction in which an OSU 50 and a plurality of ONUs 70-1 through 70-n. are mutually opposed to one another via a wavelength splitter 60 for which an AWG or a multi-port wavelength filter is utilized, an identical optical transmitter 75 which modulates broadband light having a wide spectral width in the wavelength domain is allocated to each ONU, and this broadband light (λu) is modulated by each of the ONUs and is transmitted as an upstream optical signal, and is spectrum-sliced and wavelength-division multiplexed by the wavelength splitter 60, then being transmitted to the OSU 50 (see K. Akimoto et al., “Spectrum-sliced, 25-GHz spaced, 155 Mbps×32 channel WDM access”, The 4th Pacific Rim Conference on Lasers and Electro-Optics, 2001 (CLEO/Pacific Rim 2001), Vol. 2, pp. II-556–557, hereinafter referred to as Non-Patent Reference 1). Although this structure is substantially equivalent to one in which upstream optical signals of different wavelengths are transmitted from respective ONUs, it is possible to provide a light source of the same specification to each ONU.
It should be understood that, in order to obtain modulated light of a wide optical spectral width, a method may be utilized of directly modulating a super-luminescent diode or a semiconductor optical amplifier with an electrical signal; or, alternatively, a method may be utilized of modulating the output light (broadband non-modulated light) of a semiconductor optical amplifier or of an erbium-doped optical fiber amplifier with an external modulator.
On the other hand, as another type of optical multiplexing access system, upon a star type optical access line in which a plurality of signals are combined and split via an optical power splitter, there is a system in which each ONU performs two-way transmission with the OSU by occupying a different wavelength. As a conventional such system, there is a time-division multiplexing (TDM) system which assigns a different time slot to each ONU (such as a G.983.1 B-PON system according to the ITU-T standard, or the like).
In this connection, with a laser light source which is used for creating an optical signal of a designated wavelength, when the wavelength spacing of a plurality of optical signals which are multiplexed together becomes less than, for example, a few nm, then due to temperature control and the like a wavelength control circuit becomes necessary, and it is not possible to avoid increase of the cost. When a normal distributed feedback (DFB) laser without a temperature control circuit is employed, a 20 nm wavelength spacing is established as a standard (ITU-T G.694.2) by the International Telecommunication Union Telecommunication Standardization Sector, in consideration of the fact that a wavelength variation of about ±6˜7 nm occurs over a range of about, for example, ±35° C.
When the optical wavelength-division multiplexing access system shown in FIG. 14 is constructed based upon this standard, then the wavelengths λu1 through λun of the optical carriers for upstream transmission come to be spaced at 20 nm spacings, and the wavelength band which is needed for the optical modulators 73 of the n ONUs needs to be about 20×n (nm) wide. On the other hand, when semiconductor optical amplifiers or electroabsorption optical modulators are used as the optical modulators 73, since their operating wavelength bands are about 20 to 60 nm, accordingly, if an attempt is made to manage with a single type of optical modulator, it only becomes possible to connect one to three ONUs.
Furthermore, when the optical wavelength-division multiplexing access system shown in FIG. 15 is constructed based upon the above described standard, then the wavelength band which is required for the optical transmitters 75 of the n ONUs needs to be about 20×n (nm) wide. On the other hand, when super-luminescent diodes, semiconductor optical amplifiers, or erbium-doped optical fiber amplifiers are used as the light sources for the optical transmitters 75, since their optical spectral widths are about 20 to 60 nm, accordingly, if an attempt is made to manage with a single type of light source, again, it only becomes possible to connect one to three ONUs.
On the other hand, as has been described above, if ONUs each of which occupies a different wavelength are to perform two-way transmission with an OSU upon a star type optical access line in which a plurality of signals are combined and split via an optical power splitter, then it is necessary to provide a light source of its own individual different wavelength to each ONU, and the problem arises of increase of cost due to greater multiplicity of component types in the ONUs. Furthermore, there has not been proposed detailed structure for such ONUs which are to perform two-way transmission with an OSU upon a star type optical access line in which a plurality of signals are combined and split via an optical power splitter by implementing a spectrum slicing scheme.